So 16 means mail has body and 17 means mail has body and mail was read. 4 might mean that a mail has been copied, maybe through that method of making it an item and storing it in the bag. I tried this, but when I copied the mail into my bag, it was deleted form the mailbox so I havent got a clue .....

Apparently, when sending a mail without a message, meanig it has a subject but no content, that flag is set to 4. Weird. Why not 0?

I can confirm when sending a message with subject only -> the code is 4.

So the mask MAIL_CHECK_MASK_COPIED doesn't make sense.

If you copy the mail into the bag -> the mail is deleted in the mailbox table.
If you read a mail without body it's also immediately deleted.
The same with attachments (tested with gold). If you take the gold the letter is deleted.